Epitaxial structure

ABSTRACT

The disclosure relates to an epitaxial structure. The epitaxial structure includes a substrate, an epitaxial layer, and a nanotube film. The substrate has an epitaxial growth surface. The epitaxial layer is located on the epitaxial growth surface of the substrate. The nanotube film is located between the substrate and the epitaxial layer. The nanotube film includes a number of nanotubes orderly arranged and combined with each other by ionic bonds.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.201410115667.6 filed on Mar. 26, 2014 in the China Intellectual PropertyOffice, the contents of which are incorporated by reference herein.

FIELD

The subject matter herein generally relates to epitaxial structures andmethods for making the same.

BACKGROUND

Light emitting devices such as light emitting diodes (LEDs) based ongroup III-V nitride semiconductors such as gallium nitride (GaN) havebeen put into practice.

Since wide GaN substrate cannot be produced, the LEDs have been producedon a heteroepitaxial substrate such as sapphire. The use of sapphiresubstrate is problematic due to lattice mismatch and thermal expansionmismatch between GaN and the sapphire substrate. One consequence ofthermal expansion mismatch is bowing of the GaN/sapphire substratestructure, which leads to cracking and difficulty in fabricating deviceswith small feature sizes. A solution for this is to form a plurality ofgrooves on the surface of the sapphire substrate by lithography oretching before growing the GaN layer. However, the process oflithography and etching is complex, high in cost, and will pollute thesapphire substrate.

What is needed, therefore, is to provide a method for solving theproblem discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present technology will now be described, by wayof example only, with reference to the attached figures, wherein:

FIG. 1 is a flowchart of one embodiment of a method for making anepitaxial structure.

FIG. 2 is a Scanning Electron Microscope (SEM) image of a carbonnanotube film.

FIG. 3 is a schematic structural view of a carbon nanotube segment ofthe carbon nanotube film of FIG. 2.

FIG. 4 is a schematic structural view of the carbon nanotube film ofFIG. 2.

FIG. 5 is a partially enlarged view of FIG. 4.

FIG. 6 is an SEM image of two cross-stacked carbon nanotube films.

FIG. 7 is schematic view of one embodiment of a method for stretchingthe carbon nanotube film of FIG. 4.

FIG. 8 is an SEM image of a stretched carbon nanotube film made bymethod of FIG. 7.

FIG. 9 is an SEM image of one embodiment of an alumina (Al₂O₃) layerdeposited on a carbon nanotube film not treated with oxygen plasma byatomic layer deposition (ALD).

FIG. 10 is an SEM image of one embodiment of an alumina layer depositedon a carbon nanotube film treated with oxygen plasma by atomic layerdeposition.

FIG. 11 is a transmission electron microscope (TEM) image of oneembodiment of a carbon nanotube film treated by carbon accumulation.

FIG. 12 is an SEM image of one embodiment of an alumina layer depositedon a carbon nanotube film not treated by carbon accumulation by atomiclayer deposition.

FIG. 13 is an SEM image of one embodiment of an alumina layer depositedon a carbon nanotube film treated with oxygen plasma by atomic layerdeposition.

FIG. 14 is an SEM image of one embodiment of a single alumina nanotubefilm.

FIG. 15 is an SEM image of one embodiment of two cross-stacked aluminananotube films.

FIG. 16 is a photo of an alumina nanotube film.

FIG. 17 is a schematic structural view of one embodiment of a singlenanotube film.

FIG. 18 is a schematic structural view of one embodiment of twocross-stacked nanotube films.

FIG. 19 is a schematic view of one embodiment of growing an epitaxialstructure.

FIG. 20 is a schematic structural view of one embodiment of an epitaxialstructure.

FIG. 21 is a flowchart of one embodiment of a method for making anepitaxial structure.

FIG. 22 is a schematic structural view of one embodiment of an epitaxialstructure.

DETAILED DESCRIPTION

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures and components have notbeen described in detail so as not to obscure the related relevantfeature being described. The drawings are not necessarily to scale andthe proportions of certain parts may be exaggerated to better illustratedetails and features. The description is not to be considered aslimiting the scope of the embodiments described herein.

Several definitions that apply throughout this disclosure will now bepresented.

The term “coupled” is defined as connected, whether directly orindirectly through intervening components, and is not necessarilylimited to physical connections. The connection can be such that theobjects are permanently connected or releasably connected. The term“outside” refers to a region that is beyond the outermost confines of aphysical object. The term “inside” indicates that at least a portion ofa region is partially contained within a boundary formed by the object.The term “substantially” is defined to be essentially conforming to theparticular dimension, shape or other word that substantially modifies,such that the component need not be exact. For example, substantiallycylindrical means that the object resembles a cylinder, but can have oneor more deviations from a true cylinder. The term “comprising” means“including, but not necessarily limited to”; it specifically indicatesopen-ended inclusion or membership in a so-described combination, group,series and the like. It should be noted that references to “an” or “one”embodiment in this disclosure are not necessarily to the sameembodiment, and such references mean at least one.

References will now be made to the drawings to describe, in detail,various embodiments of the present epitaxial structures and methods formaking the same.

Referring to FIG. 1, a method for making an epitaxial structure 10 ofone embodiment includes the following steps:

step (S10), providing a free standing carbon nanotube film 100, whereinthe carbon nanotube film 100 includes a plurality of carbon nanotubes104 orderly arranged and combined with each other via van der Waalsforce to form a plurality of apertures 105 extending along a lengthdirection of the plurality of carbon nanotubes 104;

step (S20), inducing defects on surfaces of the plurality of carbonnanotubes 104;

step (S30), growing a nano-material layer 110 on the surfaces of theplurality of carbon nanotubes 104 by atomic layer deposition;

step (S40), obtaining a free-standing nanotube film 114 by removing thecarbon nanotube film 100 by annealing, wherein nanotube film 114includes a plurality of nanotubes 112 orderly arranged and combined witheach other;

step (S50), placing the nanotube film 114 on an epitaxial growth surface122 of a substrate 120; and

step (S60), epitaxially growing an epitaxial layer 130 on the epitaxialgrowth surface 122 of the substrate 120.

In step (S10), the carbon nanotube film 100 is drawn from a carbonnanotube array. Referring to FIGS. 2-5, the carbon nanotube film 100 isa substantially pure structure consisting of a plurality of carbonnanotubes 104, 106, with few impurities and chemical functional groups.The carbon nanotube film 100 is a free-standing structure. The term“free-standing structure” includes that the carbon nanotube film 100 cansustain the weight of itself when it is hoisted by a portion thereofwithout any significant damage to its structural integrity. Thus, thecarbon nanotube film 100 can be suspended by two spaced supports. Themajority of carbon nanotubes 104 of the carbon nanotube film 100 arejoined end-to-end along a length direction of the carbon nanotubes 104by van der Waals force therebetween so that the carbon nanotube film 10is a free-standing structure. The carbon nanotubes 104, 106 of thecarbon nanotube film 100 can be single-walled, double-walled, ormulti-walled carbon nanotubes. The diameter of the single-walled carbonnanotubes can be in a range from about 0.5 nm to about 50 nm. Thediameter of the double-walled carbon nanotubes can be in a range fromabout 1.0 nm to about 50 nm. The diameter of the multi-walled carbonnanotubes can be in a range from about 1.5 nm to about 50 nm.

The carbon nanotubes 104, 106 of the carbon nanotube film 100 areoriented along a preferred orientation. That is, the majority of carbonnanotubes 104 of the carbon nanotube film 100 are arranged tosubstantially extend along the same direction and in parallel with thesurface of the carbon nanotube film 100. Each adjacent two of themajority of carbon nanotubes 104 are joined end-to-end by van der Waalsforce therebetween along the length direction. A minority of dispersedcarbon nanotubes 106 of the carbon nanotube film 100 may be located andarranged randomly. However, the minority of dispersed carbon nanotubes106 have little effect on the properties of the carbon nanotube film 100and the arrangement of the majority of carbon nanotubes 104 of thecarbon nanotube film 100. The majority of carbon nanotubes 104 are notabsolutely form a direct line and extend along the axial direction, someof them may be curved and in contact with each other in microcosm. Somevariations can occur in the carbon nanotube film 100.

Referring to FIG. 3, the carbon nanotube film 100 includes a pluralityof successively oriented carbon nanotube segments 108, joined end-to-endby van der Waals force therebetween. Each carbon nanotube segment 108includes a plurality of carbon nanotubes 104 parallel to each other, andcombined by van der Waals force therebetween. A thickness, length andshape of the carbon nanotube segment 108 are not limited. A thickness ofthe carbon nanotube film 100 can range from about 0.5 nanometers toabout 100 micrometers, such as 10 nanometers, 50 nanometers, 200nanometers, 500 nanometers, 1 micrometer, 10 micrometers, or 50micrometers.

Referring to FIGS. 4-5, the majority of carbon nanotubes 104 of thecarbon nanotube film 100 are arranged to substantially extend along thesame direction to form a plurality of carbon nanotube wires 102substantially parallel with each other. The minority of carbon nanotubes106 are randomly dispersed on and in direct contact with the pluralityof carbon nanotube wires 102. The extending direction of the majority ofcarbon nanotubes 104 is defined as D1, and a direction perpendicularwith Dl and parallel with the carbon nanotube film 100 is defined as D2.The carbon nanotubes 104 of each carbon nanotube wire 102 are joinedend-to-end along D1, and substantially parallel and combined with eachother along D1. The plurality of apertures 105 are defined betweenadjacent two of the plurality of carbon nanotube wires 102 or theplurality of carbon nanotubes 104.

The carbon nanotube film 100 is stretchable along D2. When the carbonnanotube film 100 is stretched along D2, the carbon nanotube film 100can maintain its film structure. A distance between adjacent two of theplurality of carbon nanotube wires 102 will be changed according to thedeformation of the carbon nanotube film 100 along D2. The distancebetween adjacent two of the plurality of carbon nanotube wires 102 canbe in a range from about 0 micrometers to about 50 micrometers. Theratio of quantity or quality between the majority of carbon nanotubes104 and the minority of dispersed carbon nanotubes 106 can be greaterthan or equal to 2:1 and less than or equal to about 6:1. The more theminority of dispersed carbon nanotubes 106, the greater the maximumdeformation of the carbon nanotube film 100 along D2. The maximumdeformation of the carbon nanotube film 100 along D2 can be about 300%.In one embodiment, the ratio of quantity between the majority of carbonnanotubes 104 and the minority of dispersed carbon nanotubes 106 isabout 4:1.

The carbon nanotube film 100 can be made by following substeps:

step (S100), providing a carbon nanotube array on a substrate; and

step (S102), drawing out the carbon nanotube film 100 from the carbonnanotube array by using a tool.

In step (S100), the carbon nanotube array includes a plurality of carbonnanotubes that are parallel to each other and substantiallyperpendicular to the substrate. The height of the plurality of carbonnanotubes can be in a range from about 50 micrometers to 900micrometers. The carbon nanotube array can be formed by the substeps of:step (S1001) providing a substantially flat and smooth substrate; step(S1002) forming a catalyst layer on the substrate; step (S1003)annealing the substrate with the catalyst layer in air at a temperatureapproximately ranging from 700° C. to 900° C. for about 30 minutes to 90minutes; step (S1004) heating the substrate with the catalyst layer to atemperature approximately ranging from 500° C. to 740° C. in a furnacewith a protective gas therein; and step (S1005) supplying a carbonsource gas to the furnace for about 5 minutes to 30 minutes and growingthe carbon nanotube array on the substrate.

In step (S1001), the substrate can be a P-type silicon wafer, an N-typesilicon wafer, or a silicon wafer with a film of silicon dioxidethereon. A 4-inch P-type silicon wafer is used as the substrate. In step(S1002), the catalyst can be made of iron (Fe), cobalt (Co), nickel(Ni), or any alloy thereof. In step (S1003), the protective gas can bemade up of at least one of nitrogen (N₂), ammonia (NH₃), and a noblegas. In step (S1005), the carbon source gas can be a hydrocarbon gas,such as ethylene (C₂H₄), methane (CH₄), acetylene (C₂H₂), ethane (C₂H₆),or any combination thereof. The carbon nanotube array formed under theabove conditions is essentially free of impurities, such as carbonaceousor residual catalyst particles.

In step (S102), the drawing out the carbon nanotube film 100 includesthe substeps of: step (S1021) selecting one or more of carbon nanotubesin a predetermined width from the carbon nanotube array; and step(S1022) drawing the selected carbon nanotubes to form nanotube segmentsat an even and uniform speed to achieve the carbon nanotube film 100.

In step (S1021), the carbon nanotubes having a predetermined width canbe selected by using an adhesive tape, such as the tool, to contact thesuper-aligned array. In step (S1022), the drawing direction issubstantially perpendicular to the growing direction of the carbonnanotube array. Each carbon nanotube segment includes a plurality ofcarbon nanotubes parallel to each other.

In one embodiment, during the drawing process, as the initial carbonnanotube segments are drawn out, other carbon nanotube segments are alsodrawn out end-to-end due to van der Waals force between ends of adjacentsegments. This process of drawing helps provide a continuous and uniformcarbon nanotube film 100 having a predetermined width can be formed.

The width of the carbon nanotube film 100 depends on a size of thecarbon nanotube array. The length of the carbon nanotube film 100 can bearbitrarily set as desired. In one useful embodiment, when the substrateis a 4-inch P-type silicon wafer, the width of the carbon nanotube film100 can be in a range from about 0.01 centimeters to about 10centimeters. The thickness of the carbon nanotube film 100 can be in arange from about 0.5 nanometers to about 10 micrometers.

Furthermore, at least two carbon nanotube films 100 can be stacked witheach other or two or more carbon nanotube films 100 can be locatedcoplanarly and combined by only the van der Waals force therebetween. Asshown in FIG. 6, two carbon nanotube films 100 are stacked with eachother, and the majority of carbon nanotubes 104 of the two carbonnanotube films 100 are substantially perpendicular with each other.

Furthermore, in one embodiment, step (S10) further includes stretchingthe carbon nanotube film 100 along D2 so that the apertures 105 havelarger width. As shown in FIG. 7, the stretching the carbon nanotubefilm 100 includes: fixing two opposite sides of the carbon nanotube film100 on two spaced elastic supporters 200 so that a portion of the carbonnanotube film 100 are suspended between the two elastic supporters 200,wherein two elastic supporters 200 are parallel with each other andextend along D2; stretching the two elastic supporters 200 along D2 toobtain a stretched carbon nanotube film. As shown in FIG. 8, thestretched carbon nanotube film has increased apertures. The two elasticsupporters 200 can be elastic rubber, springs, or elastic bands. Thespeed of stretching the two elastic supporters 200 is less than 10centimeters per second. The area of the carbon nanotube film 100 can beincreased by stretching along D2.

Furthermore, in one embodiment, step (S10) can further include treatingthe carbon nanotube film 100 with organic solvent so that the apertures105 have larger width. The organic solvent can be volatile, such asethanol, methanol, acetone, dichloroethane, chloroform, or mixturesthereof. In one embodiment, the organic solvent is ethanol. The treatingthe carbon nanotube film 100 with organic solvent can be performed byapplying the organic solvent to entire surface of the carbon nanotubefilm 100 suspended on a frame or immersing the entire carbon nanotubefilm 100 with the frame in an organic solvent.

In one embodiment, the treating the carbon nanotube film 100 withorganic solvent includes soaking a suspended carbon nanotube film 100with an atomized organic solvent at least one time. In one embodiment,the soaking a suspended carbon nanotube film 100 can include steps of:providing a volatilizable organic solvent; atomizing the organic solventinto a plurality of dispersed organic droplets; and spraying the organicdroplets onto the surface of the suspended carbon nanotube film 100 andthe organic droplets gradually penetrating onto the carbon nanotubes ofthe carbon nanotube film 100, thereby making the suspended carbonnanotube film 100 be soaked at least one time by the organic droplets,and then make the carbon nanotube film shrink into a treated carbonnanotube film. The organic droplets are tiny organic solvent dropssuspended in surrounding. The organic solvent can be atomized into theorganic droplets by ultrasonic atomization method, high pressureatomizing method or other methods.

The organic solvent can be alcohol, methanol, acetone, acetic acid, andother volatilizable solvents. During the spraying process, a pressure isproduced, when the organic droplets are sprayed, the pressure is smalland can't break the carbon nanotube film 100. The diameter of eachorganic droplet is larger than or equal to 10 micrometers, or less thanor equal to 100 micrometers, such as about 20 micrometers, 50micrometers. Thus, an interface force is produced between the carbonnanotube film 100 and the organic droplets. The interface force canensure that the carbon nanotube film 100 is shrunk and the carbonnanotubes in the carbon nanotube film 100 are dispersed more uniformly.

The organic solvent is volatile and easy to be volatilized. When theorganic droplets are sprayed onto the carbon nanotube film 100 and thenpenetrated into the carbon nanotube film 100, the organic droplets arethen volatilized, and the carbon nanotube segments 108 loosely arrangedin the carbon nanotube film 100 are tightly shrunk. The diameter of eachorganic droplet is larger than or equal to 10 micrometers, or less thanor equal to 100 micrometers, the soaked scope of the carbon nanotubesegment of the carbon nanotube film 100 is limited by the small diameterof each organic droplet. Thus, diameters of the carbon nanotube segments108 of the carbon nanotube film 100 can be shrunk into less than orequal to 10 micrometers, the carbon nanotube segments 108 aresubstantially invisible using naked eyes in the treated carbon nanotubefilm. The carbon nanotube film 100 is original black or grey. However,after the soaking with an atomized organic solvent, the carbon nanotubefilm 100 is shrunk into the treated carbon nanotube film which is moretransparent.

In step (S20), the carbon nanotube film 100 can be suspended and treatedby oxidization or carbon accumulation to induce defects. In oneembodiment, part of the carbon nanotube film 100 is suspended byattaching on a frame and oxidized by oxygen plasma treating.

In the process of treating the carbon nanotube film 100 by oxygenplasma, the carbon nanotubes 104, 106 of the carbon nanotube film 100are destroyed to form a plurality of dangling bond. In the process ofoxygen plasma, the carbon nanotube film 100 can be connected to a powerand has an electric potential so that the carbon nanotube film 100 canbe bombard by the oxygen plasma more strongly. When nano-material layer110 are forming on the surface of the carbon nanotubes 104, 106 byatomic layer deposition, the atoms of the nano-material layer 110 canstack on the surface of the carbon nanotubes 104, 106 layer upon layerto form a compact nano-material layer 110 with high strength. Also, thethickness of the nano-material layer 110 is controllable so that anano-material layer 110 with a thickness in nano-scale can be obtained.In the process of oxygen plasma, the flow rate of the oxygen gas can bein a range from about 30 sccm to about 60 sccm, the pressure of theoxygen gas can be in a range from about 8 Pa to about 12 Pa, thetreating time can be in a range from about 8 seconds to about 12seconds, and the treating power can be in a range from about 20 W toabout 30 W. In one embodiment, the flow rate of the oxygen gas is about50 sccm, the pressure of the oxygen gas is about 10 Pa, the treatingtime is about 10 seconds, and the treating power is about 25 W. As shownin FIG. 9, if the carbon nanotube film is not treated with oxygenplasma, the alumina layer deposited on the carbon nanotube film byatomic layer deposition is a plurality of discontinuous particles.However, as shown in FIG. 10, if the carbon nanotube film is treatedwith oxygen plasma, the alumina layer deposited on the carbon nanotubefilm by atomic layer deposition is a continuous layer structure.

In the process of carbon accumulation, a carbon layer is coated on thesurface of the carbon nanotubes 104, 106. The method of carbonaccumulation can be physical vapor deposition (PVD), chemical vapordeposition (CVD), or spraying. In the process of carbon accumulation, anelectric current can be supplied to flow through the carbon nanotubefilm 100 to produce heat to heat the carbon nanotube film 100 itself. Inone embodiment, the carbon layer is coated on the surface of the carbonnanotubes 104, 106 by magnetron sputtering. In the magnetron sputtering,the current can be in a range from about 100 mA to about 200 mA, thepressure can be in a range from about 0.05 Pa to about 0.2 Pa, the flowrate of Ar can be in a range from about 5 Pa to about 15 sccm, and thesputtering time can be in a range from about 1.5 minutes to about 7.5minutes. In one embodiment, the current is about 150 mA, the pressure isabout 0.1 Pa, the flow rate of Ar is about 10 sccm, and the sputteringtime is about 5 minutes. As shown in FIG. 11, the carbon layer is coatedon the surface of the carbon nanotubes of the carbon nanotube film 100by magnetron sputtering.

The carbon layer includes a plurality of carbon particles to form thedefects on the surface of the carbon nanotubes 104, 106. Thus, whennano-material layer 110 are forming on the surface of the carbonnanotubes 104, 106 by atomic layer deposition, the atoms of thenano-material layer 110 can stack on the surface of the carbon nanotubes104, 106 layer upon layer to form a compact nano-material layer 110 withhigh strength. Also, the thickness of the nano-material layer 110 iscontrollable so that a nano-material layer 110 with a thickness innano-scale can be obtained. The thickness of the nano-material layer 110can be in a range from about 10 nanometers to about 30 nanometers. Ifthe carbon nanotube film 100 is not treated by carbon accumulation, thealumina layer deposited on the carbon nanotube film by atomic layerdeposition can form a continuous layer structure only at the thicknessabove 30 nanometers. If the thickness of the alumina layer deposited onthe carbon nanotube film by atomic layer deposition is smaller than 30nanometers, the alumina layer is a plurality of discontinuous particlesattached on the surface of the carbon nanotubes 104, 106. Thus, thealumina layer cannot form a compact layer structure. As shown in FIG.12, if the carbon nanotube film 100 is not treated with carbonaccumulation, the alumina layer deposited on the carbon nanotube film byatomic layer deposition is a plurality of discontinuous particles.However, as shown in FIG. 13, if the carbon nanotube film is treatedwith carbon accumulation, the alumina layer deposited on the carbonnanotube film by atomic layer deposition is a continuous layerstructure.

In step (S30), the source material of the atomic layer deposition can beselected according to the material of the nanotubes 112. For example,when the nanotubes 112 are metal oxide nanotubes 112, the sourcematerial of the atomic layer deposition includes metal organic compoundand water, and the carrier gas is nitrogen gas. The thickness of thenano-material layer 110 can be in a range from about 10 nanometers toabout 100 nanometers. In one embodiment, the thickness of thenano-material layer 110 is in a range from about 20 nanometers to about50 nanometers. The nano-material layer 110 can be coated on a surface ofa single carbon nanotube to form a continuous layer structure andenclose the single carbon nanotube therein. The nano-material layer 110can also be coated on a surface of two or more than two carbon nanotubesto form a continuous layer structure and enclose the two or more thantwo carbon nanotubes therein. The nano-material layer 110 can form aplurality of nanotubes 112 after the carbon nanotubes therein areremoved because the nano-material layer 110 is a compact continuouslayer structure. The plurality of nanotubes 112 can be combined witheach other to form a free-standing nanotube film 114. The nanotube 112can be a single linear nanotube or a branch nanotube. The material ofthe nano-material layer 110 can be metal oxide, metal nitride, metalcarbide, silicon oxide, silicon nitride, or silicon carbide.

In one embodiment, an alumina layer is deposited on the carbon nanotubefilm 100 by atomic layer deposition, the source materials of the atomiclayer deposition are trimethylaluminum and water, and the carrier gas isnitrogen gas. The alumina layer is deposited on the carbon nanotube film100 by following steps:

step (S301), suspending a portion of the carbon nanotube film 100 in avacuum chamber of a atomic layer deposition device; and step (S302),alternately introducing trimethylaluminum and water in to the chamber ofthe atomic layer deposition device.

In step (S301), the carbon nanotube film 100 is attached on a frame sothat a portion of the carbon nanotube film 100 is suspended, and thenplaced into the vacuum chamber with the frame together. The frame can bea metal or ceramic frame. Because the carbon nanotube film 100 is freestanding, the carbon nanotube film 100 can be direct placed on twospaced supporters located in the vacuum chamber. When the elasticsupporters 200 mentioned above are made of thermostable material, thestretched carbon nanotube film 100 and the elastic supporters 200 can beplaced in the vacuum chamber together.

In step (S302), the carrier gas is nitrogen gas. The flow rate of thecarrier gas is about 5 sccm. The alternately introducingtrimethylaluminum and water includes following steps:

step (S3021), first evacuating the vacuum chamber to a pressure of about0.23 Torr; step (S3022), introducing trimethylaluminum in to the vacuumchamber until the pressure of the vacuum chamber rises to about 0.26Torr;

step (S3023), second evacuating the vacuum chamber to the pressure ofabout 0.23 Torr;

step (S3024), introducing water in to the vacuum chamber until thepressure of the vacuum chamber rise to about 0.26 Torr;

step (S3025), third evacuating the vacuum chamber to the pressure ofabout 0.23 Torr; and

step (S3026), repeating step (S3022) to step (S3025) to start anothercycle.

In each cycle, the second evacuating the vacuum chamber to the pressureof about 0.23 Ton takes about 25 seconds, and the third evacuating thevacuum chamber to the pressure of about 0.23 Ton takes about 50 seconds.The deposition velocity of the alumina layer is about 0.14 nm/cycle. Thethickness of the alumina layer can be controlled by the cycle number.

In step (S40), the carbon nanotube film 100 coated with nano-materiallayer 110 is annealed to remove the carbon nanotube film 100 and obtainthe plurality of nanotubes 112. The plurality of nanotubes 112 areorderly arranged and combined with each other to form the free-standingnanotube film 114. The annealing can be performed in oxygen atmosphereand at a temperature in a range from about 500° C. to about 1000° C. Inone embodiment, the carbon nanotube film 100 coated with nano-materiallayer 110 is suspended in a quartz tube filled with air and heated to550° C.

Referring to FIGS. 2 and 14, the single alumina nanotube film and thesingle carbon nanotube film have substantially the same structure.Referring to FIGS. 6 and 15, the two cross-stacked alumina nanotubefilms and the two cross-stacked carbon nanotube films have substantiallythe same structure. FIG. 16 shows that the alumina nanotube film is afree-standing nanotube film.

Referring to FIG. 17, the nanotube film 114 includes a plurality ofnanotubes 112 orderly arranged and combined with each other. In oneembodiment, the plurality of nanotubes 112 are arranged to extendsubstantially along the same direction. The plurality of nanotubes 112are spaced from each other or in direct contact with each other. Aplurality of apertures 116 are defined by the plurality of spacednanotubes 112. Thus, the nanotube film 114 is a patterned mask. Theplurality of apertures 116 extends through the nanotube film along thethickness direction. The plurality of apertures 116 can be a holedefined by several adjacent nanotubes 112, or a gap defined by twosubstantially parallel nanotubes 112 and extending along axial directionof the nanotubes 112. The hole shaped apertures 116 and the gap shapedapertures 116 can exist in the patterned nanotube film 114 at the sametime. The sizes of the apertures 116 can be different. The average sizeof the apertures 116 can be in a range from about 10 nanometers to about500 micrometers. For example, the sizes of the apertures 116 can beabout 50 nanometers, 100 nanometers, 500 nanometers, 1 micrometer, 10micrometers, 80 micrometers, or 120 micrometers. The smaller the sizesof the apertures 116, the less dislocation defects will occur during theprocess of growing the epitaxial layer 130. In one embodiment, the sizesof the apertures 116 are in a range from about 10 nanometers to about 10micrometers. The adjacent nanotubes 112 are combined with each other byionic bonds at the contacting surface and internal communicated. Atleast part of the plurality of nanotubes 112 extend from a first side ofthe nanotube film 114 to a second side opposite to the first side. Themajority of nanotubes 112 of the nanotube film 114 are arranged tosubstantially extend along the same direction and in parallel with thesurface of the nanotube film 114. A minority of dispersed nanotubes 112of the nanotube film 114 may be arranged randomly and in direct contactwith the adjacent nanotubes 112. Thus, the nanotube film 114 is afree-standing structure. The thickness of the wall of each nanotube 112can be in a range from about 10 nanometers to about 100 nanometers. Theinside diameter of each nanotube 112 can be in a range from about 1nanometer to about 100 nanometers. As shown in FIG. 18, two nanotubefilms 114 can be stacked with each other. The extending directions ofnanotubes 112 in the two stacked nanotube films 114 are perpendicularwith each other. The two stacked nanotube films 114 are combined witheach other by ionic bonds.

The nanotube film 114 can be used as a mask for growing the epitaxiallayer 130. The mask is the nanotube film 114 sheltering a part of theepitaxial growth surface 122 and exposing another part of the epitaxialgrowth surface 122. Thus, the epitaxial layer 130 can grow from theexposed epitaxial growth surface 122. The nanotube film 114 can form apatterned mask on the epitaxial growth surface 122 because the nanotubefilm 114 defines a plurality of apertures 116. After the nanotube film114 is placed on the epitaxial growth surface 122 of the substrate 120,part of the epitaxial growth surface 122 is sheltered by the nanotubefilm 114, and other part of the epitaxial growth surface 122 is exposedfrom the plurality of apertures 116. A dutyfactor of the nanotube film114 is an area ratio between the sheltered epitaxial growth surface 122and the exposed epitaxial growth surface 122. The dutyfactor of thenanotube film 114 can be in a range from about 1:100 to about 100:1. Forexample, the dutyfactor of the nanotube film 114 can be about 1:10, 1:2,1:4, 4:1, 2:1, or 10:1. In one embodiment, the dutyfactor of thenanotube film 114 is in a range from about 1:4 to about 4:1. Comparedwith lithography or etching, the method of forming a nanotube film 114as mask is simple, low in cost, and will not pollute the substrate 120.

In step (S50), the epitaxial growth surface 122 can be used to grow theepitaxial layer 130. The epitaxial growth surface 122 is a clean andsmooth surface. The substrate 120 can be a single-layer structure or amulti-layer structure. If the substrate 120 is a single-layer structure,the substrate 120 can be a single crystal structure having a crystalface used as the epitaxial growth surface 122. If the substrate 120 is amulti-layer structure, the substrate 120 should include at least onelayer having the crystal face. The material of the substrate 120 can beGaAs, GaN, AN, Si, SOI (silicon on insulator), SiC, MgO, ZnO, LiGaO₂,LiAlO₂, or Al₂O₃. The material of the substrate 120 can be selectedaccording to the material of the epitaxial layer 130. The epitaxiallayer 130 and the substrate 120 should have a small lattice mismatch anda thermal expansion mismatch. The size, thickness, and shape of thesubstrate 120 can be selected according to need. In one embodiment, thesubstrate 120 is a sapphire substrate.

After the nanotube film 114 is placed on the epitaxial growth surface122 of the substrate 120, the plurality of nanotubes 112 extends along adirection parallel with the epitaxial growth surface 122. The pluralityof nanotubes 112 can also be arranged to extend along thecrystallographic orientation of the substrate 120 or along a directionwhich forms an angle with the crystallographic orientation of thesubstrate 120. Part of the epitaxial growth surface 122 is sheltered bythe nanotube film 114, and part of the epitaxial growth surface 122 isexposed from the plurality of apertures 116. In one embodiment, thenanotube film 114 is placed on the entire epitaxial growth surface 122.

In step (S60), the nanotube film 114 is used as a mask for growing theepitaxial layer 130. The mask can shelter a part of the epitaxial growthsurface 122 and exposing another part of the epitaxial growth surface122. Thus, the epitaxial layer 130 can grow from the exposed epitaxialgrowth surface 122. Compared to lithography or etching, the method ofusing the nanotube film 114 as mask is simple, low in cost, and will notpollute the substrate 120.

The epitaxial layer 130 is a single crystal layer grown on the epitaxialgrowth surface 122 by epitaxy growth method. The material of theepitaxial layer 130 can be the same as or different from the material ofthe substrate 120. If the epitaxial layer 130 and the substrate 120 arethe same material, the epitaxial layer 130 is called a homogeneousepitaxial layer. If the epitaxial layer 130 and the substrate 120 havedifferent material, the epitaxial layer 130 is called a heteroepitaxialepitaxial layer. The material of the epitaxial layer 130 can besemiconductor, metal or alloy. The semiconductor can be Si, GaAs, GaN,GaSb, InN, InP, InAs, InSb, AlP, AlAs, AlSb, AN, GaP, SiC, SiGe, GaMnAs,GaAlAs, GaInAs, GaAlN, GaInN, AlInN, GaAsP, InGaN, AlGaInN, AlGaInP,GaP:Zn, or GaP:N. The metal can be aluminum, platinum, copper, orsilver. The alloy can be MnGa, CoMnGa, or Co₂MnGa. The thickness of theepitaxial layer 130 can be prepared according to need. The thickness ofthe epitaxial layer 130 can be in a range from about 100 nanometers toabout 500 micrometers. For example, the thickness of the epitaxial layer130 can be about 200 nanometers, 500 nanometers, 1 micrometer, 2micrometers, 5 micrometers, 10 micrometers, or 50 micrometers.

Referring to FIG. 19, step (S60) includes the following substeps:

step (S601), nucleating on the epitaxial growth surface 122 and growinga plurality of epitaxial crystal grains 132 along the directionsubstantially perpendicular to the epitaxial growth surface 122;

step (S602), forming a continuous epitaxial film 134 by making theepitaxial crystal grains 132 grow along the direction substantiallyparallel to the epitaxial growth surface 122; and

step (S603), forming the epitaxial layer 130 by making the epitaxialfilm 134 grow along the direction substantially perpendicular to theepitaxial growth surface 122.

In step (S601), the epitaxial crystal grains 132 grow from the exposedpart of the epitaxial growth surface 122 and through the apertures 116.The process of the epitaxial crystal grains 132 growing along thedirection substantially perpendicular to the epitaxial growth surface122 is called vertical epitaxial growth.

In step (S602), the epitaxial crystal grains 132 are joined together toform an integral structure (the epitaxial film 134) to cover thenanotube film 114. The epitaxial crystal grains 132 grow and form aplurality of caves to enclose the nanotubes 112 of the nanotube film114. The inner wall of the caves can be in contact with or spaced fromthe nanotubes 112, depending on whether the material of the epitaxialfilm 134 and the nanotubes 112 have mutual infiltration. Thus, theepitaxial film 134 defines a patterned depression on the surfaceadjacent to the epitaxial growth surface 122. The patterned depressionis related to the patterned nanotube film 114. If nanotube film 114includes a layer of parallel and spaced nanotubes 112, the patterneddepression is a plurality of parallel and spaced grooves. If thenanotube film 114 includes a plurality of nanotubes 112 crossed orweaved together to form a net, the patterned depression is a groovenetwork including a plurality of intersected grooves. The nanotube film114 can prevent lattice dislocation between the epitaxial crystal grains132 and the substrate 120 from growing. The process of epitaxial crystalgrains 132 growing along the direction substantially parallel to theepitaxial growth surface 122 is called lateral epitaxial growth.

In step (S603), the epitaxial layer 130 is obtained by growing for along duration of time. Because the nanotube film 114 can prevent thelattice dislocation between the epitaxial crystal grains 132 and thesubstrate 120 from growing, the epitaxial layer 130 has fewer defectstherein.

The epitaxial layer 130 can be grown by a method such as molecular beamepitaxy, chemical beam epitaxy, reduced pressure epitaxy, lowtemperature epitaxy, select epitaxy, liquid phase deposition epitaxy,metal organic vapor phase epitaxy, ultra-high vacuum chemical vapordeposition, hydride vapor phase epitaxy, or metal organic chemical vapordeposition (MOCVD).

In one embodiment, the substrate 120 is a sapphire substrate, and theepitaxial layer 130 is grown on the sapphire substrate by MOCVD method.The nitrogen source gas is high-purity ammonia (NH₃), the Ga source gasis trimethyl gallium (TMGa) or triethyl gallium (TEGa), and the carriergas is hydrogen (H₂). The alumina nanotube film 114 is placed on anepitaxial growth surface of the sapphire substrate 120. The growth ofthe epitaxial layer 130 includes the following steps:

step (a), locating the sapphire substrate 120 with the alumina nanotubefilm 114 thereon into a reaction chamber, heating the sapphire substrateto about 1100° C. to about 1200° C., introducing the carrier gas, andbaking the sapphire substrate 120 for about 200 seconds to about 1000seconds;

step (b), growing a low-temperature GaN buffer layer with a thickness ofabout 10 nanometers to about 50 nanometers by cooling down thetemperature of the reaction chamber to a range from about 500° C. to650° C. in the carrier gas atmosphere, and introducing the Ga source gasand the nitrogen source gas at the same time;

step (c), stopping the flow of the Ga source gas while maintaining theflow of the carrier gas and nitrogen source gas atmosphere, increasingthe temperature to a range from about 1100° C. to about 1200° C., andannealing for about 30 seconds to about 300 seconds; and

step (d), maintaining the temperature of the reaction chamber in a rangefrom about 1000° C. to about 1100° C., and reintroducing the Ga sourcegas to grow the high quality epitaxial layer 130.

Furthermore, a step of placing a second nanotube film 114 on a surfaceof the epitaxial layer 130 spaced from the substrate 120 and growing asecond epitaxial layer 130 on the surface of the epitaxial layer 130 canbe performed.

Referring to FIG. 20, an epitaxial structure 10 provided in oneembodiment includes a substrate 120, a nanotube film 114, and anepitaxial layer 130. The substrate 120 has an epitaxial growth surface122. The nanotube film 114 is located on the epitaxial growth surface122 and defines a plurality of apertures 116. The epitaxial layer 130 islocated on the nanotube film 114 and contacts the epitaxial growthsurface 122 through the apertures 116. The epitaxial layer 130 defines aplurality of caves adjacent to and oriented to the epitaxial growthsurface 122. The caves can be blind holes or grooves. The caves and theepitaxial growth surface 122 cooperatively form a sealed chamber toreceive the nanotube film 114 therein. The inner wall of the caves canbe spaced from the nanotubes 112 of the nanotube film 114.

Referring to FIG. 21, a method for making an epitaxial structure 20 ofone embodiment includes the following steps:

step (S10), providing a free standing carbon nanotube film 100, whereinthe carbon nanotube film 100 includes a plurality of carbon nanotubes104 orderly arranged and combined with each other via van der Waalsforce to form a plurality of apertures 105 extending along a lengthdirection of the plurality of carbon nanotubes 104;

step (S20), inducing defects on surfaces of the plurality of carbonnanotubes 104 by suspending and treating the carbon nanotube film 100;

step (S30), growing a nano-material layer 110 on the surfaces of theplurality of carbon nanotubes 104 by atomic layer deposition;

step (S40), obtaining a free-standing nanotube film 114 by removing thecarbon nanotube film 100 by annealing, wherein nanotube film 114includes a plurality of nanotubes 112 orderly arranged and combined witheach other;

step (S50), suspending the nanotube film 114 above an epitaxial growthsurface 122 of a substrate 120; and

step (S60), epitaxially growing an epitaxial layer 130 on the epitaxialgrowth surface 122 of the substrate 120.

The method for making an epitaxial structure 20 is similar to the methodfor making an epitaxial structure 10 described above except that in step(50), the nanotube film 114 is suspended above and spaced from theepitaxial growth surface 122. A distance between the nanotube film 114and the epitaxial growth surface 122 can be in a range from about 10nanometers to about 500 micrometers. In one embodiment, the distancebetween the nanotube film 114 and the epitaxial growth surface 122 is ina range from about 50 nanometers to about 100 micrometers, such as about10 micrometers.

Referring to FIG. 22, an epitaxial structure 20 provided in oneembodiment includes a substrate 120, a nanotube film 114, and anepitaxial layer 130. The epitaxial structure 20 is similar to theepitaxial structure 10 described above except that the nanotube film 114is spaced from the epitaxial growth surface 122 and fully enclosed bythe epitaxial layer 130.

The embodiments shown and described above are only examples. Even thoughnumerous characteristics and advantages of the present technology havebeen set forth in the foregoing description, together with details ofthe structure and function of the present disclosure, the disclosure isillustrative only, and changes may be made in the detail, including inmatters of shape, size and arrangement of the parts within theprinciples of the present disclosure up to, and including, the fullextent established by the broad general meaning of the terms used in theclaims.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. The description and the claims drawn to a method may includesome indication in reference to certain steps. However, the indicationused is only to be viewed for identification purposes and not as asuggestion as to an order for the steps.

What is claimed is:
 1. An epitaxial structure comprising: a substratehaving an epitaxial growth surface; an epitaxial layer located on theepitaxial growth surface of the substrate; and a nanotube film locatedbetween the substrate and the epitaxial layer, wherein the nanotube filmcomprises a plurality of nanotubes orderly arranged and combined witheach other by ionic bonds.
 2. The epitaxial structure of claim 1,wherein adjacent two of the plurality of nanotubes are internalcommunicated at contacting surface.
 3. The epitaxial structure of claim1, wherein the plurality of nanotubes are parallel to the epitaxialgrowth surface.
 4. The epitaxial structure of claim 1, wherein twostacked nanotube films are located between the substrate and theepitaxial layer.
 5. The epitaxial structure of claim 4, whereinnanotubes of the two stacked nanotube films are arranged toperpendicular with each other.
 6. The epitaxial structure of claim 4,wherein the two stacked nanotube films are combined with each other byionic bonds.
 7. The epitaxial structure of claim 1, wherein a thicknessof wall of each of the plurality of nanotubes is in a range from about10 nanometers to about 100 nanometers.
 8. The epitaxial structure ofclaim 1, wherein at least part of the plurality of nanotubes extend froma first side of the nanotube film to a second side opposite to the firstside.
 9. The epitaxial structure of claim 1, wherein a length of asingle one of the plurality of nanotubes is the same as a length or awidth of the nanotube film.
 10. The epitaxial structure of claim 1,wherein a material of the plurality of nanotubes is selected from thegroup consisting of metal oxide, metal nitride, metal carbide, siliconoxide, silicon nitride, and silicon carbide.
 11. The epitaxial structureof claim 1, wherein the nanotube film defines a plurality of aperturesto expose part of the epitaxial growth surface of the substrate.
 12. Theepitaxial structure of claim 11, wherein part of the epitaxial layerextend through the plurality of apertures to in contact with theepitaxial growth surface of a substrate.
 13. The epitaxial structure ofclaim 1, wherein the epitaxial layer defines a plurality of caves andthe plurality of nanotubes are embedded in the plurality of caves. 14.The epitaxial structure of claim 1, wherein the nanotube film is afree-standing structure.
 15. An epitaxial structure comprising: asubstrate having an epitaxial growth surface; an epitaxial layer locatedon the epitaxial growth surface of the substrate; and a nanotube filmlocated between the substrate and the epitaxial layer, wherein thenanotube film comprises a plurality of nanotubes made of a materialselected from the group consisting of metal oxide, metal nitride, metalcarbide, silicon oxide, silicon nitride, and silicon carbide.
 16. Theepitaxial structure of claim 15, wherein the nanotube film is afree-standing structure.